The explanation Cochrane proposes is surprisingly straightforward for so unusual an outcome: that visual input comes in through the retinas of one girl, reaches her thalamus, then takes two different courses, like electricity traveling along a wire that splits in two. In the girl who is looking at the strobe or a stuffed animal in her crib, the visual input continues on its usual pathways, one of which ends up in the visual cortex. In the case of the other girl, the visual stimulus would reach her thalamus via the thalamic bridge, and then travel up her own visual neural circuitry, ending up in the sophisticated processing centers of her own visual cortex. Now she has seen it, probably milliseconds after her sister has.

The results of the test did not surprise the family, who had long suspected that even when one girl’s vision was angled away from the television, she was laughing at the images flashing in front of her sister’s eyes. The sensory exchange, they believe, extends to the girls’ taste buds: Krista likes ketchup, and Tatiana does not, something the family discovered when Tatiana tried to scrape the condiment off her own tongue, even when she was not eating it.

Even knowing about the tests and what Cochrane believed, I listened to the family’s stories with some amount of skepticism. Perhaps they were imagining it or exaggerating for the sake of a good story. Then in one of the many idle moments of the five days I spent with the family, the girls were watching television, and I absent-mindedly gave Tatiana’s foot, which Krista could not see, a little tickle. She turned to me and smiled, and then Krista spoke: “Now do me,” she said. Had she felt the sensation but wanted the emotional experience of knowing that she, too, was receiving that kind of playful attention?